1. Field of the Invention
This invention relates generally to tension leg platforms, and more particularly to tether or tendon installation and tensioning systems for tension leg platforms.
2. Brief Description of the Prior Art
In the offshore oil and gas industry, a tension leg platform or "TLP" is often used in deepwater production activities. The tension leg platform has a buoyant hull which supports a work deck from which the drilling and production operations are conducted. The platform hull is moored to a foundation on the ocean floor by a set of elongate tethers or tendons which extend from the foundation and are secured to the buoyant hull under tension. The tethers or tendons are kept in tension by the buoyancy of the platform hull and maintain the hull at a significantly greater draft than if it were free floating. Tidal motion and wave action are compensated for by lateral movement of the platform and tethers. Vertical movements, normally associated with heave, pitch and roll motions of the sea are eliminated by the combined buoyancy of the platform and tethers.
One of the problem areas in conventional tension leg platform design and operation is the system for installing and tensioning of the tethers. In many conventional TLP systems, the tethers are installed by lowering them to the ocean floor through columns on the platform hull. To accomplish the installation, the tethers are made up of threaded tubular segments which are secured together section by section as the tether is lowered. This method is time consuming and increases the vulnerability of the platform to adverse weather conditions during the installation process.
Because of the expense involved in providing costly tensioning equipment for each tendon being worked on, the normal practice in prior art tension leg platform installation methods is to "lock-off" on a single tether (tendon) per platform corner. However, this practice often causes large stresses in the tethers (tendons) resulting from resonant vertical oscillations of the platform which may occur when it is only partially secured.
There are several patents which disclose various tether installation apparatus and methods.
Hunter, U.S. Pat. No. 4,320,993 discloses a connector apparatus for connecting a tension leg platform mooring tether to a subsea foundation. The apparatus comprises a plug positioned on the lower end of the mooring tether for mating with a receptacle positioned in the foundation so that the plug is positioned in the receptacle to maintain the mooring tether in connection with the foundation with the plug being removable from the receptacle when desired.
Collipp, U.S. Pat. No. 4,620,820 discloses a method an apparatus for anchoring a tension leg platform using an anchor having an upper and lower assembly. The upper assembly is operatively connected to the lower ends of the tethers forming the tension legs of the platform and acts to space and align each tether in a vertical manner when the upper assembly is connected to the lower assembly which has been previously secured to the ocean floor by anchor piles.
Gunderson, U.S. Pat. No. 4,881,852 discloses a system for securing and tensioning the tethers of a tension leg platform. The top ends of the tethers are positioned a distance below the bottom of the platform hull and a removable tensioning tool is situated within the hull for lowering a tether extender to each tether and then biasing each tether and tether extender unit upward to tension the tether.
Peppel, U.S. Pat. No. 4,943,188 discloses a rotating lug anchor connector for anchoring the tether of a tension leg platform to the sea floor. The connector includes a latch assembly mounted on the lower end of the tether having lugs which are slidably received in slots in a foundation receptacle and engaged on notches in the foundation receptacle upon upward movement. The tether is released by lowering it and moving the lugs into alignment with the slots.
The present invention is distinguished over the prior art in general, and these patents in particular by a method and apparatus for installing and tensioning mooring tethers or tendons between a subsea foundation and a tension leg platform. Load bearing porch members secured on the platform columns near the hull receive the threaded upper end of tethers which are engaged at their lower ends in the foundation. Movable hoists on the platform deck position the upper ends of the tethers on the porches. A platform arrester through which the hoist lines pass is connected with each hoist to move with the platform during vertical motions thereof and selectively engage the hoist lines to allow only downward movement of the platform arrester relative to the hoist lines. A motorized tether tensioning tool connected to each hoist line has a lock-down nut at its lower which releasably connects to the upper end of each tether such that the nut is vertically movable on the tether. The upper ends of the tethers are positioned on the porches and nominal constant tension is applied to the tethers while the platform arresters allow only downward movement relative to the hoist line until platform trim is within predetermined limits. The tensioning tools then run the nuts down on the tethers to engage them on the porches and apply tension in the tethers between the porches and foundation and are thereafter removed whereby tension loading in each tether is transmitted to the platform such that the total platform tension load is equally distributed between the whole tether complement.